The present invention relates to a data processing apparatus having a direct memory access (DMA) function.
In data processing apparatus including a computer such as a minicomputer or a microcomputer used for protection or control of a system such as an electric power transmission system, a water supply system, or a rolling mill, information is transferred between an external device and a memory device in the data processing apparatus, either by the use of an input/output register under the control of the central processing unit (CPU), or by DMA, i.e., independently of the CPU. In the former system, the burden on the CPU is greater so that the performance of the CPU is lowered. The latter system is free from such a problem.
A DMA has two modes of operation. In a first mode, called a burst mode, operation of the CPU is interrupted while data is exchanged by the DMA. In a second mode, called a cycle steal mode, execution of the program by the CPU is carried out in parallel with the exchange of the data by DMA.
The information to be transferred between the external device and the memory device includes:
(a) information concerning the status of the system to be protected or controlled. PA0 (b) information concerning setting values used as reference values for data processing, and PA0 (c) an output of the contents of the memory device.
The information transferred between the external device and the memory device is required to be monitored for ensuring satisfactory operation and maintenance of the system to be protected. The monitoring has to be carried out without affecting the speed of data processing, and without interfering with protection and control functions, and hence independently of the operation of the CPU.